1. Field
The present invention relates to a data conversion/output apparatus and, more particularly, to a data conversion/output apparatus for converting data sensed by each sensor into a digital value and outputting the digital value in a sensor array constituted by laying out a plurality of sensors for detecting light or a capacitance.
2. Description of the Related Art
As a sensor array constituted by laying out a plurality of sensors for detecting light or a capacitance, many sensor arrays have recently been developed for a photosensor or fingerprint sensor which has a matrix of a plurality of pixels 60 each bearing a sensor 61 for detecting light or a fingerprint shape and reads an image or fingerprint, as shown in FIG. 10. In these sensor arrays, analog data detected by a sensor in each pixel must be converted into digital data and output outside a pixel array 50. Various data conversion/output apparatuses for converting data sensed by each sensor into a digital value and outputting it have been proposed.
FIG. 11 shows the first prior art of a conventional data conversion/output apparatus (see, e.g., D. Renshow et al., “ASIC VISION”, Digest of IEEE Custom Integrated Circuits Conference, 1990). In the prior art of FIG. 11, sensors 61 and switches 63 are integrated in respective pixels 60, and these pixels 60 are arrayed in a matrix. The switch 63 in each pixel 60 is controlled by a signal from a column decoder 72, and connected to a data bus 74 shared by the pixels 60 on each row.
The data bus 74 is connected to a corresponding switch 75, and to an A/D converter 79 via the switch 75. The switch 75 is controlled by a signal from a row decoder 73.
In this data conversion/output apparatus, light or the like is detected by the sensors 61. The address of a column to be read is input to the column decoder 72, and the column decoder 72 sends a signal for closing the switches 63 of pixels 60 on the column whose address has been input. The sensors 61 on the selected column output detected analog data to the data bus 74 shared by each row. Then, the address of a row to be read is input to the row decoder 73, and the row decoder 73 sends a signal for closing the switches 75 connected to the data bus 74.
The selected data bus 74 is connected to the A/D converter 79, and the analog data output to the data bus 74 is input to the A/D converter 79. The A/D converter 79 converts the input analog data into digital data and outputs it outside as sensed data. By performing this operation for all the columns and rows, all read data can be converted into digital data and output outside.
FIG. 12 shows the second prior art (see, e.g., A. Simoni et al., “A Digital Camera for Machine Vision”, Conference on Industrial Electronics, Control and Instrumentation, 1994). In the prior art of FIG. 12, the data bus 74 shared by pixels 60 on each row as the first prior art is connected to a corresponding comparison circuit 78, and the other input of the comparison circuit 78 is connected to the output of a D/A converter 80. The input of the D/A converter 80 is connected to the output of a counter 76, and the output of the counter 76 is connected to each latch circuit 77. As a read signal to the latch circuit 77, an output from the comparison circuit 78 is input. An output from the latch circuit 77 is output outside via a corresponding switch 75, and the switch 75 is controlled by a signal from a row decoder 73.
In the data conversion/output apparatus, light or the like is detected by the sensors 61. The address of a column to be read is input to the column decoder 72, and the column decoder 72 sends a signal for closing the switches of pixels on the column whose address has been input. The sensors 61 on the selected column output detected analog data to the comparison circuit 78 via the data bus 74 shared by each row. The counter 76 increments data from a minimum value to a maximum value, and outputs the data. The output from the counter is input to the D/A converter 80, and the D/A converter 80 outputs analog data which increases stepwise in correspondence with the count value.
The comparison circuit 78 compares the analog data output from the D/A converter 80 with the analog data output from the sensors, and only when the data coincide with each other in magnitude, sends a read signal to the latch circuit 77. The latch circuit 77 receives an output from the counter 76, and when the read signal is sent from the comparison circuit 78, latches a count value at that time. Then, the address of a row to be read is input to the row decoder 73, and the row decoder 73 sends a signal for closing the switches 75 connected to the data bus 74. An output from the latch circuit 77 on the selected row is output outside as sensed data via the switch 75. By performing this operation for all the columns and rows, all read data can be converted into digital data and output outside.
These conventional data conversion/output apparatuses suffer the following problems. The first prior art requires a long time in order to convert and output data of all the pixels because data detected by pixels are converted by the A/D converter one by one. Analog data is transmitted via a long path and many elements, so detected data may be easily degraded by noise or the like.
In the second prior art, the dynamic range, resolution, and precision of data to be output are limited by the precision of the D/A converter because reference analog data to be compared with analog data detected by the sensor is generated by the D/A converter. Output data may also be degraded by the precision of the comparison circuit for comparing analog data or variations between comparison circuits.